The invention relates to a method of bonding a sintered friction material to a steel reinforcing cup to produce a brake or clutch lining assembly.
In the past, friction material has been attached to steel reinforcing cup by either a mechanical or brazed bond.
U.S. Pat. No. 2,784,105 discloses a typical mechanical bond. The mechanical bond is created by crimping the sides of the retainer cup around the friction material. Unfortunately, under some operational load conditions, a separation can occur between the friction material and the reinforcing cup.
In an effort to provide a better bond between the friction material and the reinforcing cup, a screen was welded to the reinforcing cup. A green preform of friction material was placed in the reinforcing cup. When the friction material is compacted to a desired density, the green preform flows around the screen to form a matrix. Then the friction material and the reinforcing cup are both heated in an oven where sintering hardens and strengthens the friction material. Unfortunately, often times after sintering, the reinforcing cup is distorted and the entire brake lining assembly must be discarded. In addition, the effective oven capacity is reduced because all the friction material, cup and screen must be heated to establish the mechanical bond.
In order to reduce the cost of a friction lining assembly, it was suggested that the screen could be eliminated if a metallurgical bond could be created between the friction material and the steel reinforcing cup. U.S. Pat. Nos. 3,528,807 and 3,761,256 disclose a method of producing such a metallurgical bond through the use of a brazing material. During the sintering of the friction material in the oven, the brazing material forms an alloy between the friction material and reinforcing cup. Unfortunately, the cost of the brazing materials and extra labor involved in this process has limited customer acceptance of this type of bonding.
Later, as disclosed in copending U.S. Pat. application Ser. No. 683,460, an economical method of bonding was devised for metallurgically bonding a sintered friction material to a reinforcing cup. In this method, the sintered friction material and the reinforcing cup are uniformly heated in an oven until a predetermined temperature is present throughout the entire assembly. When this predetermined temperature is reached, the assembly is moved to a press and a single blow applied to densify the friction material. Because of the temperature and force of the single blow a weld is created along the interface of the friction material and the reinforcing cup. Unfortunately, the microstructures of some friction materials are changed when the single blow is struck while the friction materials are at a high temperature, thereby rendering the end product unacceptable for all brake or clutch requirements. In addition, warping of the welded assembly is a problem when the assembly cools from the high temperature which still remains after the application of the press blow.